So each device that OBP can be expected to talk to must have
its very own firmware, which describes how the device is operated. This
firmware is used to teach OBP how to read a block off a disk on a
particular SCSI controller type, for example.
The net result is that if a device lacks the appropriate
OpenBoot firmware, it cannot be used for booting the OS or be used as
the console.
But this is not so much a limitation as it may seem. All SPARC
systems have some sort of onboard boot device (which does have the
appropriate firmware) and
console.
On PCI UltraSPARC systems, Linux happens to support several
PCI devices even if they lack the special firmware. So you can place
a cheap Ethernet card into one of the PCI slots and Linux will happily
use it. You just can't boot from it.